Method and device for synchronising mobile terminals on a radio channel in direct mode

ABSTRACT

The invention concerns a method whereby each of the communication using the channel comprises transmission between mobile terminals ( 11 - 13 ) of signals distributed into traffic time intervals and uniformly distributed control time intervals during which one of the mobile terminals transmits synchronising and signalling signals. Some at least of the terminals determine standby time intervals aligned on the control intervals and maintained after communication, and are set in reception on the channel during the standby time intervals, with idle periods between said standby time intervals.

[0001] Most professional radiocommunication systems offer afunctionality called the direct mode or walkie-talkie mode, which allowsmobile radio terminals to communicate with one another outside of thecoverage of a network.

[0002] This functionality has several uses: it makes it possible to makeup for insufficient coverage of the network; it ensures continuity ofservice in the event of failure or of destruction of the fixedinstallations of the network; finally, it makes it possible to carry outnetwork independent operations when the required.

[0003] The functions available in direct mode are in general morerestricted than those available in network mode. One of the functionswhich is most often absent is the terminal standby function.

[0004] The standby function consists, for a terminal which is notcommunicating, in placing itself from time to time in a receive state soas to examine the state of the channel and to detect any calls relevantto it. For the remainder of the time, the terminal is sleeping so as tolimit its energy consumption.

[0005] In former analog systems, the absence of a standby function didnot present serious drawbacks. Indeed, the consumption of the receptionpart of the terminal in the absence of any signal was relatively small.Moreover, most of the terminals were automobile mobile terminals whoseconsumption was not critical.

[0006] The digital systems have greater consumption in the receptionpart, due to the necessary digital processing. With the development ofhandheld terminals, this makes terminal standby extremely useful.

[0007] In the network mode, the infrastructure controls the instants atwhich the terminals are woken up since it fixes the temporal structurefor use of the channels. Synchronization of standby is therefore easilyachieved. However, in direct mode, a form of cooperation must be set upbetween the terminals since the infrastructure may be out of range ornonexistent. Most of the standby mechanisms used in direct modegenerally lead to a loss of the start of the communication or to arelatively long setup time.

[0008] European patent 0 786 174 describes a system in which basestations of the network supervise the standby function of the terminalsboth in respect of the radio resources of the network and in respect ofthe direct mode. This approach is suitable only in the particular casewhere the terminals are within range of base stations. This case is notthe most frequent.

[0009] An object of the present invention is to provide an efficientstandby mechanism in direct mode, i.e. one which offers a relativelylarge amount of standby and guarantees a relatively short communicationsetup time.

[0010] It is also desirable for the mechanism to be well adapted tovarious modes of communication setup: mode without acknowledgment (themost common in direct mode), or modes offering a possibility ofacknowledgment, for example by going off-hook after ringing.

[0011] The invention thus proposes a method of synchronizing mobileterminals on a direct mode radio channel, wherein each communicationoccupying the channel comprises the transmission between mobileterminals of signals distributed into traffic time slots and intoregularly dispersed control time slots during which one of the mobileterminals transmits synchronization and signaling signals. According tothe invention, some at least of the terminals determine standby timeslots aligned with the control slots and continuing after thecommunication, and place themselves in a receive state on the channelduring the aligned standby slots, with idle periods between said standbyslots.

[0012] Thus, at the end of the communication, all the terminals whichhave participated in the communication, or which have synchronizedthemselves with the communication without participating therein, canimplement the standby mechanism with mutually synchronized wakeupinstants, based on the synchronization of the previous communication.

[0013] This synchronous standby mechanism will thus allow themdefinitely to detect and to monitor any communication set up on thischannel and using the same synchronization.

[0014] In a preferred embodiment of the method, some at least of theterminals further determine standby time slots which are desynchronizedwith respect to the control slots, having a different periodicity fromthat of the control slots and continuing after the communication, andalso place themselves in the receive state on the channel during thedesynchronized standby slots.

[0015] This second offset monitoring by the terminals on standby allowsfast detection of any communication not using the same time setting.

[0016] In response to the detection on the channel of signals other thansynchronization and signaling signals in a standby slot, a mobileterminal remains in the receive state on the channel and determines newstandby time slots if it detects synchronization and signaling signals.

[0017] This allows the terminal to acquire the synchronization of thedirect mode channel if it was previously using another synchronization,in particular if it was disconnected.

[0018] Another aspect of the present invention pertains to a controldevice for a mobile radiocommunication terminal, the terminal comprisingmeans of transmission and of reception on a direct mode radio channel,whereby each communication occupying said channel comprises thetransmission between mobile terminals of signals distributed intotraffic time slots and into regularly dispersed control time slotsduring which one of said mobile terminals transmits synchronization andsignaling signals. This control device comprises means of activation ofthe means of reception on the channel during standby slots and ofdeactivation of the means of reception on the channel outside thestandby slots, and means for determining time slots, continuing afterthe communication, which are aligned with control slots positioned inthe course of a communication by detection of synchronization signalstransmitted by another terminal and for including said aligned slots inthe standby slots.

[0019] Another aspect of the present invention pertains to a mobileradiocommunication terminal including such a control device.

[0020] Other features and advantages of the invention will becomeapparent from the following description of non-limiting exemplaryembodiments, with reference to the appended drawings, in which:

[0021]FIG. 1 is a diagram of a radiocommunication system forimplementing the invention;

[0022]FIG. 2 is a schematic diagram of a mobile terminal according tothe invention;

[0023] FIGS. 3 to 9 are charts illustrating the organization of a directmode channel into elements of multiframes and the manner of operation ofterminals on this channel.

[0024] The radiocommunication system represented in FIG. 1 comprises anetwork whose infrastructure comprises one or more base stations 10.These base stations 10 are capable of setting up radio channels withmobile terminals (11-13).

[0025] The system considered is a frequency division multiple access(FDMA) system. A certain number of carrier frequencies are devoted tonetwork mode communications in which the radio transmissions occurbetween a base station 10 and one or more mobile terminals 11-13.

[0026] Another frequency channel is reserved for direct modecommunications between the terminals, as illustrated between theterminals 12 and 13 in FIG. 1. A direct mode communication on thefrequency channel may occur when one at least of the relevant terminals13 is outside the coverage area 15 of the network, but this is notcompulsory.

[0027] In the exemplary embodiment described below, the direct modechannel is organized into successive multiframes each comprising aplurality of 360 ms elements, each composed of 18 time slots of 20 ms(FIG. 3).

[0028] When a communication is set up and activated on the direct modechannel, sixteen of these time slots are devoted to the continuoustransmission of signals representing coded speech or more generally usertraffic (traffic slots), whereas the other two time slots serve for theperiodic transmission of signals representing synchronization andsignaling information (control slots), as shown by FIG. 3. In each ofFIGS. 3 to 9, the time slots drawn with cross-hatching are controlslots.

[0029] When a dialog is necessary between two terminals, for example forthe initiation of a communication in acknowledged mode, the multiframeelement still comprises two time slots for the periodic transmission ofthe synchronization and signaling information by one or other of theterminals, but the remaining time slots are shared statically betweenthe two directions of communication. This is illustrated by FIG. 4 in aparticular case where the sharing is performed by alternation of blocksof n=3 time slots of transmission by the terminal transmitting thesynchronization and signaling information and of blocks of n=3 timeslots of reception by this terminal, with a vacant time slot with eachchange of direction.

[0030]FIG. 2 diagrammatically shows the means with which a terminal11-13 for communicating on the direct mode channel are equipped (themeans relating to the network mode are not drawn).

[0031] A duplexer 20 is linked to the antenna 21 of the terminal so asto separate the transmission pathway and the reception pathway which arerespectively provided with a radio transmitter 22 and with a radioreceiver 23. A controller 24 ensures the sequencing of the transmitter22 and of the receiver 23 in accordance with the multiframe structureillustrated by FIGS. 3 to 9.

[0032] At the output of the radio receiver 23, a demultiplexer 25distributes the demodulated signal between a module 26 for the controltime slots and a vocoder 27 for the traffic time slots. Likewise, in thetransmission direction, the controller 24 instructs a multiplexer 28 tosend the output signals from the vocoder 27 to the radio transmitter 22in the traffic time slots, and synchronization and signaling informationemanating from the module 26 in the control time slots if the terminalis the one which ensures the broadcasting of this information.

[0033] A time base 29 associated with a crystal oscillator supplies thecontroller 24 with a clock enabling it to obtain the timing of themultiframes and of the 20 ms time slots.

[0034] To synchronize the terminal with respect to other terminalsengaged in a communication, the module 26 detects the synchronizationand/or signaling information dispatched in the control time slots, andsupplies a corresponding time marker to the controller 24 which can theninstruct the other facilities of the terminal in alignment with thestructure of the multiframes.

[0035] When a direct mode communication is set up and activated, thechannel is occupied by 320 ms sequences of coded speech, intercut with40 ms sequences, used either for the broadcasting of synchronization andchannel state information (characteristics of the communication), or toallow return signaling from the participating terminals to thetransmitter terminal (see FIG. 3).

[0036] A terminal which has decided not to participate in thecommunication, nevertheless has knowledge of the synchronization of thecommunication which has been detected by its module 26. Its controller24 can then implement a standby mechanism such that it periodicallyactivates the radio receiver 23 so as to monitor part of the informationbroadcast and thus to ascertain whether the communication is continuingor has terminated. In the example illustrated by FIG. 5, the terminalwakes up during a standby slot of 20 ms every 360 ms, aligned with thefirst control slot of the multiframe element.

[0037] Thus, at the end of the communication, all the terminals whichhave participated in the communication or which have synchronized withthe communication without participating therein can continue to applythe same standby mechanism with mutually synchronized wakeup instants,based on the synchronization of the previous communication.

[0038] This synchronization lasts as long as the drifts of the localtime bases 29 of each of the terminals do not generate a large offset.The terminals will then be said to be in a synchronized standby state.This standby mechanism will then allow them definitely to detect and tomonitor any communication set up on the direct mode channel and usingthe same synchronization.

[0039] In the event of migration or of power-up of a terminal, itsstandby characteristics are probably not synchronous with those of theterminals which have become neighbors. This terminal will be said to besearching for synchronization, from the point of view of its mutuallysynchronized neighbors.

[0040] In this case, if a new communication is set up, the periodicinstants of wakeup of the migrating terminal do not coincide with theinstants of broadcasting of the synchronization and channel stateinformation.

[0041] However, its receiver 23 can, during its wakeup periods, detectthe presence of an energy level above a given threshold, therebyindicating that the channel is occupied by signals relating to acommunication in direct mode. The controller 24 thus maintains theterminal in the receive state until a synchronization sequence isdetected.

[0042] If after decoding of the signaling information accompanying thissynchronization sequence, the terminal determines that the communicationis relevant to it, the controller 24 will activate the receiver 23and/or the transmitter 22 in a corresponding manner.

[0043] Otherwise, the terminal places itself in a standby statesynchronized with the other terminals, in accordance with the mechanismset forth hereinabove. The latter case is that illustrated by FIG. 6.

[0044] If the terminal maintained in the receive state after detectionof signals other than synchronization and signaling signals in a standbyslot does not succeed in subsequently detecting synchronization andsignaling signals, it returns to its standby state preceding the expiryof a timeout whose duration may be of the order of that of themultiframe element.

[0045] The latter mechanism operates correctly when the communicationoccupies the entire channel since the continuous transmission by one ofthe terminals allows definite detection of energy level outside of thecontrol slots.

[0046] However, it is not necessarily suitable during the setup phasesfor a communication in acknowledged mode. In this case, the initiatingterminal broadcasts synchronization and channel state information with aperiodicity identical to that used for the communications which havebeen set up, but the remainder of the time comprises a dialog (or anattempted dialog) with the terminal to be contacted, and thereforecomprises regular phases of transmission and of reception (or ofawaiting reception), as is illustrated in FIG. 4. The time required forthe acquisition of synchronization by a non-synchronized terminal, i.e.until one of the wakeup instants coincides with a transmission phase,may then become random, or be excessively lengthened.

[0047] This drawback is even more serious when the terminal seeking tosynchronize turns out to be precisely the called terminal.

[0048] The terminal on standby should then perform a further periodicmonitoring desynchronized with respect to the duration of an element ofthe multiframe, for example by monitoring the channel for an additionalstandby slot of 20 ms every 300 ms.

[0049] Thus, whatever case is envisaged, one of the desynchronizedstandby slots will quickly coincide with a transmission phase (eitherperiodic synchronization and channel state information or an element ofa setup dialog). The terminal can then trigger the time-delayed searchfor synchronization, and thus attain a synchronized standby state, asillustrated by FIG. 7 where the upper part shows the standby slotsaligned with the periodicity of the elements of the control slots andthe lower part shows the desynchronized standby slots.

[0050] In the case of a change of speaker, there is also a period duringwhich transmission is not continued, and the mechanism illustrated byFIG. 7 also improves the speed of synchronization in this transientsituation.

[0051] It is noted that in order to favor the detection of energy by theterminal when searching for synchronization, the transmitter cancontinue to transmit in all the blocks of n time slots which arereserved for it in the multiframe outside of active communication (seeFIG. 4). In this case, having regard to the static allocation of theblocks reserved for the initiating terminal, the periodicity of thedesynchronized monitorings may be optimized at (360±n×p×20) ms (n and pintegers). Specifically, it is not necessary to scan all the blocks ofthe multiframe.

[0052] This mechanism composed of two standby time slots, called alignedstandby slots in respect of those corresponding to synchronous standby(every 360 ms) and desynchronized standby slots in respect of theothers, allows the terminals in all cases:

[0053] to be in a state of synchronized standby with the neighboringterminals as soon as a communication has taken place; and

[0054] to quickly acquire the same standby synchronization duringmigration or power-up.

[0055] The amount of standby offered by this mechanism is compatiblewith a good energy saving of the battery since the terminal is in thereceive state on average 2 times 20 ms every 360 ms, i.e. around 11% ofthe time.

[0056] Preferably, during the setting up of a communication innon-acknowledged mode, the initiating terminal does not begin sendingcoded speech immediately, since the terminals which are able toparticipate in the communication and hence to receive the coded speechmay be on standby. An initial setup sequence allowing the terminals onstandby to detect the presence of a communication is thereforeperformed.

[0057] Several strategies are possible, according to whether one favors(1) the certainty that the relevant terminals will be present or (2) asfast as possible a switch to communication.

[0058] In approach (1), the terminal wishing to initiate thecommunication begins to transmit synchronization and signalinginformation throughout the duration separating it from the end of thenext aligned standby slot used to wake up all the neighboring terminals,this being the one which it would itself have used for its next wakeupif the communication had not been initiated. As all the terminals wakeup at this instant, their switch to communication is ensured (FIG. 8).The same holds for most of the terminals searching for synchronization,whose standby slots, aligned or desynchronized, will often fall duringthe initial period of transmission of the synchronization and signalinginformation.

[0059] In this case, the communication setup time is 220 ms on averageand 400 ms in the worst case.

[0060] It will, however, be noted that the coding of the speech canbegin before the transmission of the first traffic time slot, therebyreducing the delay perceived by the user.

[0061] In approach (2), at the cost of a small probability of losing thefirst few syllables of the communication, it is possible to furtherreduce the time to switch to communication by taking account of thedesynchronized standby slots. The terminal wishing to initiate thecommunication in non-acknowledged mode transmits synchronization andsignaling information for a duration T, equal to the minimum of theduration t remaining until the next aligned standby slot, and of a fixedduration T₀ (FIG. 9).

[0062] In this case, all the terminals whose desynchronized standby slotoccurs in the course of this transmission of duration T are woken up intime, whereas the other terminals will enter the communication after thefirst time slot devoted to the periodic sending of the synchronizationand signaling information of the multiframe, i.e. at the latest (360—T₀)ms after the start of sending of the coded speech (the traffic loss isthen (360—T₀) ms).

[0063] It is possible to vary T₀ so as to guarantee the best compromisebetween the average duration of communication setup and the averageduration of coded speech loss at the start of communication.

[0064] During the setup of a communication in acknowledged mode, it isnecessary to ensure the waking up of the terminal to be contacted. Theapproach (1) above should therefore be adopted. If the terminal to becontacted was not synchronized and if none of the standby slots coincidewith the period of initial transmission of the synchronization andsignaling information, the transmitter repeats the setup message atpredetermined instants corresponding to the blocks of n=3 transmissiontime slots of FIG. 4, so that it will certainly be received by theterminal on non-synchronized standby at the latest after three secondarystandby periods, i.e. 900 ms after the start of setup in the embodimentillustrated by FIG. 7.

1. A method of synchronizing mobile terminals (11-13) on a direct mode radio channel, wherein each communication occupying the channel comprises the transmission between mobile terminals of signals distributed into traffic time slots and into regularly dispersed control time slots during which one of the mobile terminals transmits synchronization and signaling signals, characterized in that some at least of the terminals determine standby time slots aligned with the control slots and continuing after the communication, and place themselves in a receive state on the channel during the aligned standby slots, with idle periods between said standby slots.
 2. The method as claimed in claim 1, wherein some at least of the terminals (11-13) further determine standby time slots desynchronized with respect to the control slots, having a different periodicity from that of the control slots and continuing after the communication, and also place themselves in the receive state on the channel during the desynchronized standby slots.
 3. The method as claimed in any one of the preceding claims, wherein, in response to the detection on the channel of signals other than synchronization and signaling signals in a standby slot, a mobile terminal (11-13) remains in the receive state on the channel and determines new standby time slots if it detects synchronization and signaling signals.
 4. The method as claimed in claim 3, wherein said mobile terminal (11-13) remains in the receive state on the channel until the expiry of a timeout if it does not detect any synchronization and signaling signals.
 5. The method as claimed in any one of the preceding claims, wherein, to initiate a new communication, a mobile terminal (11-13) transmits synchronization and signaling signals until the next aligned standby slot.
 6. The method as claimed in claim 2, wherein, to initiate a new communication in non-acknowledged mode, a mobile terminal (11-13) transmits synchronization and signaling signals for a duration equal to the minimum between a predetermined duration and the duration remaining to run until the end of the next aligned standby slot.
 7. A control device for a mobile radiocommunication terminal, the terminal comprising means of transmission (22) and of reception (23) on a direct mode radio channel, whereby each communication occupying said channel comprises the transmission between mobile terminals (11-13) of signals distributed into traffic time slots and into regularly dispersed control time slots during which one of said mobile terminals transmits synchronization and signaling signals, the control device comprising means of activation of the means of reception on the channel during standby slots and of deactivation of the means of reception on the channel outside the standby slots, characterized in that it comprises means for determining time slots, continuing after the communication, which are aligned with control slots positioned in the course of a communication by detection of synchronization signals transmitted by another terminal and for including said aligned slots in the standby slots.
 8. The control device as claimed in claim 7, wherein time slots desynchronized with respect to the control slots positioned in the course of the communication, having a different periodicity from that of the control slots and continuing after the communication are further included in the standby slots.
 9. The control device as claimed in claim 7 or 8, wherein the means of activation and of deactivation are arranged to activate the means of reception on the channel (23) until the detection of synchronization and signaling signals in response to the detection on the channel of signals other than synchronization and signaling signals in a standby slot.
 10. The control device as claimed in claim 9, wherein the means of activation and of deactivation are arranged to limit the activation of the means of reception on the channel (23) to a predetermined timeout duration in response to the detection on the channel of signals other than synchronization and signaling signals in a standby slot.
 11. The control device as claimed in any one of claims 7 to 10, further comprising means (24, 26) of initiating a new communication, to instruct the means of transmission on the channel (22) to transmit synchronization and signaling signals until the next aligned standby slot.
 12. The control device as claimed in claim 8, further comprising means (24, 26) of initiating a new communication in non-acknowledged mode to instruct the means of transmission on the channel (22) to transmit synchronization and signaling signals for a duration equal to the minimum between a predetermined duration (T₀) and the duration (t) remaining to run until the next aligned standby slot.
 13. A mobile radiocommunication terminal, comprising means of transmission (22) and of reception (23) on a direct mode radio channel and a control device, whereby each communication occupying said channel comprises the transmission between mobile terminals (11-13) of signals distributed into traffic time slots and into regularly dispersed control time slots during which one of said mobile terminals transmits synchronization and signaling signals, the control device comprising means of activation of the means of reception on the channel during standby slots and of deactivation of the means of reception on the channel outside the standby slots, characterized in that it comprises means for determining time slots, continuing after the communication, which are aligned with control slots positioned in the course of a communication by detection of synchronization signals transmitted by another terminal and for including said aligned slots in the standby slots.
 14. The mobile terminal as claimed in claim 13, wherein time slots desynchronized with respect to the control slots positioned in the course of the communication, having a different periodicity from that of the control slots and continuing after the communication are further included in the standby slots.
 15. The mobile terminal as claimed in claim 13 or 14, wherein the means of activation and of deactivation are arranged to activate the means of reception on the channel (23) until the detection of synchronization and signaling signals in response to the detection on the channel of signals other than synchronization and signaling signals in a standby slot.
 16. The mobile terminal as claimed in claim 15, wherein the means of activation and of deactivation are arranged to limit the activation of the means of reception on the channel (23) to a predetermined timeout duration in response to the detection on the channel of signals other than synchronization and signaling signals in a standby slot.
 17. The mobile terminal as claimed in any one of claims 13 to 16, further comprising means (24, 26) of initiating a new communication, to instruct the means of transmission on the channel (22) to transmit synchronization and signaling signals until the next aligned standby slot.
 18. The mobile terminal as claimed in claim 14, further comprising means (24, 26) of initiating a new communication in non-acknowledged mode to instruct the means of transmission on the channel (22) to transmit synchronization and signaling signals for a duration equal to the minimum between a predetermined duration (T₀) and the duration (t) remaining to run until the next aligned standby slot. 